For the optimal control of an internal combustion engine, a variety of sensor units are necessary which are used to detect, for example, pressures or mass flow rates of combustion air supplied to the internal combustion engine, or of exhaust gas or recirculated exhaust gas. Since the functionality of the sensor unit has a great influence on safe and low-emission operation of the internal combustion engine, it is necessary to be able to diagnose errors of the sensor units so that appropriate responses may be initiated in the event of errors or malfunctions of the sensor units. In principle, it is possible to provide each sensor unit in duplicate, so that an error of one sensor unit may be detected by comparing the signals of both sensor units. However, this is expensive, requires considerable space, and increases the weight of a motor vehicle. In addition, the increase in the number of sensors is accompanied by an increased probability of failure of the overall system.
For the above reasons, attempts have been made to largely dispense with a redundant sensor system and use other methods for error diagnosis. For example, the sensor units may be monitored electrically, thus allowing an unplugged cable or a short circuit to be detected.
It is also possible to check various sensor units for plausibility at given operating points of the internal combustion engine, for example, by comparing the signals of an ambient pressure sensor, a boost pressure sensor, and an exhaust gas back-pressure sensor when the internal combustion engine is at a standstill. This type of error detection may be easily carried out, but has the disadvantage of a time-limited monitoring range.
Plausibility checking of a first sensor is also possible by modeling expected values for the first sensor based on signals of other sensors, and comparing the expected values to the actual value from the first sensor. However, an interaction with calibration functions may occur. In addition, it is not always possible to unambiguously identify a defective sensor of a sensor system.
In the method stated at the outset, an output signal of the sensor unit is compared to a setpoint value which is specified independently from the output signal. This method may also be referred to as a “physical signal range check.” The defect in a sensor unit is detected when a measured value is outside of a physically meaningful value range for use in a motor vehicle. This error diagnosis may be easily carried out, but heretofore has been suitable only for detecting limited error patterns.
A method is discussed in german patent document DE 10 2005 025 884 A1 for correcting a signal of a sensor, in which at least one characteristic variable of the sensor signal is compared to a reference value. The sensor signal is corrected as a function of the comparison result. A value for the at least one characteristic variable of the sensor signal which is derived from the sensor signal is formed as the reference value. Thus, the reference value is specified as a function of the output signal of the sensor unit.